Immunomodulation by behavioural conditioning

The contemporary surge in studies of behaviourally conditioned immunomodulation following Ader and Cohen (1975) was preceded by Pavlovian conditioning conducted in the Soviet Union during the 1920s and 1950s. Data from these studies provided a tenable hypothesis for immunomodification using classical conditioning processes. In particular a variant of Pavlovian conditioning, the conditioned taste aversion (CTA) paradigm, has proved very robust; here a novel gustatory experience (CS) is paired with an aversive physiological event (UCS) and subsequent gustatory avoidance of the CS alone is measured. Similar procedures have also been successfully applied to the conditioned alteration of cellular immune responses. Studies demonstrating behaviourally conditioned effects on the immune system are reviewed. More recently, conditioned immunoenhancement studies have demonstrated that taste aversion can induce a conditioned increase in the helper: suppressor T cell subset ratio in rats, and a depressed DTH response in mice. Mediating mechanisms for the conditioned effects are examined, in particular the roles of adrenocorticotropin (ACTH) and beta-endorphin (BEP), and the routes of communication between the CNS and immune system. Clinical applications of immunomodification may also be demonstrated by the potential therapeutic efficacy of both conditioned immunosuppression and immunoenhancement.     

Effects of taste aversion conditioning on the primary antibody response to sheep red blood cells and Brucella abortus in the albino rat

It has been shown that use of the saccharin/cyclophosphamide taste aversion paradigm produced conditioned immunosuppression as well as saccharin avoidance after two post-conditioning exposures to the aversive stimulus [3,18]. In the present study the effects of saccharin/cyclophosphamide conditioning on the primary humoral antibody response to two antigens: sheep red blood cells (SRBC), a T-cell dependent antigen and Brucella abortus (B. abortus), a T-cell independent antigen, were examined. In addition the effects of a third exposure to the aversive stimulus, saccharin, on conditioned immunosuppression were examined. The results indicate that the target cell of taste aversion conditioning, in relation to immune dysfunction, could be the T-lymphocyte and that conditioned immunosuppression is dependent on the presence of the behavioral response. These results constitute a replication of previous studies [3,18] and provide evidence which indicates that behaviorally conditioned immunosuppression is a consequence of the parameters of taste aversion conditioning.     

Disruption of conditioned taste aversion by the combined effects of LiCl and ECS

Rats were taught a conditioned taste aversion by allowing them to drink sucrose (CS) for 5 min and 30 min later poisoning them with LiCl (UCS). Experimental animals were given ECS (80 mA for 250 msec) at 0, 15, 20, 25, 27.5, 30, 32.5, 35, 40, or 45 min after the CS. Only animals given ECS both within the CS-UCS interval and in close temporal proximity to the UCS (within 5 min) showed a significant, although limited, disruption of learning. At least two explanations are possible. The first is that apart from its toxicity, LiCl may also possess amnesic properties which interact with those of the ECS. Alternatively, ECS may have impaired the ability of the animals to fully experience the lithium-induced illness.     

Enhancement of Pavlovian conditioned immunosuppression in rats

The goal of this study was to define conditions under which conditioned immunosuppression may be observed reliably. In three experiments, rats were exposed to a gustatory conditioned stimulus (CS) paired with cyclophosphamide (US), which induces immunosuppression and malaise. In Experiment 1, a single pairing of the CS with low, medium, or high doses of cyclophosphamide in separate groups produced no reliable conditioned immunosuppression even though conditioned taste aversion was observed in groups trained with high and medium doses of CY. Experiment 2 replicated the lack of effect following a single pairing of the CS with the medium dose of cyclophosphamide but demonstrated that three pairings are sufficient to induce conditioned immunosuppression. Experiment 3 demonstrated that significant immunosuppression is observable following a single CS-US pairing if the CS is presented in compound with a previously nonreinforced CS during training, an effect reminiscent of supernormal conditioning. These findings indicate that conditioned immunosuppression effects can be enhanced in magnitude through the use of certain procedural techniques.     

Further examination of ontogenetic limitations on conditioned taste aversion

This study was to resolve a discrepancy in the literature as to the capability of infant rats in acquiring conditioned taste aversion. Previous studies had indicated that during the 1st postnatal week, an aversion to saccharin could be conditioned when paired with lithium chloride (LiCl). Analogous conditioning with sucrose did not seem to occur until the end of the 2nd postnatal week, however, even though sucrose is discriminated from water and preferred before then. We observed that 5- and 9-day old pups express conditioned taste aversion to both saccharin and sucrose flavors that previously were paired with illness induced by LiCl. This learning occurred only when several hours separated cannulation and conditioning. A number of other factors that seemed likely to determine this early learning were found to have no effect. Thus it appears that rats can learn taste aversions very early in life, but only under certain circumstances. The results are discussed with reference to Vogt and Rudy's (1984) conclusions on the ontogeny of taste guided behaviors in the rat.     

Familiarization and cross-familiarization of wheel running and LiCl in conditioned taste aversion

Familiarization with an unconditioned stimulus (US) interferes with the learning of subsequent Pavlovian conditioned associations. We conducted a series of experiments exploiting this US preexposure effect to elucidate the underlying mechanism of conditioned taste aversion in rats induced by voluntary wheel running. Experiment 1 demonstrated that running-induced taste aversion was alleviated if rats had sufficient experience of running in advance. In Experiments 2A and 2B, preexposure to wheel running had no effect on subsequent taste aversion conditioning by lithium chloride (LiCl) injection. In Experiment 3, however, we observed a weak partial interference from the LiCl injection pretreatment to the subsequent conditioning by wheel running US. This US crossover effect suggests the possibility that wheel running and LiCl injection share a common or similar physiological mechanism in inducing taste aversion.     

Plasma corticosterone levels as an index of the strength of illness induced taste aversions

Data are presented which demonstrate the task generality of pituitary-adrenal changes that accompany avoidance conditioning. In two experiments a conditioned aversion to milk was established by pairing it with lithium chloride (LiCl). In Experiment 1 conditioned pituitary-adrenal activation occurred when, in a conflict situation, animals reexperienced milk that had earlier been paired with LiCl. The relationship between the strength of aversion and corticosterone levels was such that animals showing the greatest avoidance showed the largest elevations in plasma corticosterone. In Experiment 2 this behavior/steroid relationship was manipulated. Dexamethasone (DEX) pretreatment on the day of conditioning was used to attenuate the conditioned aversion. Compared to saline (SAL) controls when reexposed to milk, DEX animals showed an attenuated aversion (i.e., drank more) and a smaller conditioned response (i.e., less adrenocortical activity). The reduction of conditioned elevation in corticosterone was not due to any residual effects of DEX at the time of testing (Experiment 3). Plasma levels of corticosterone represent an index for assessing the strength of illness induced conditioned taste aversions.     

Disruption of conditioned taste aversion by ECS: the role of lithium chloride

Rats were taught a conditioned taste aversion (CTA) by pairing a 10% sucrose solution (CS) with lithium chloride (LiCl)-induced poisoning (UCS). The CS-UCS interval was 30 min. The LiCl dose (20 ml/kg) was either strong (0.15 M) or weaker (0.075 M). Electroconvulsive shock (ECS) (80 mA for 600 msec) was interpolated within the CS-UCS interval at either 15 or 30 min. ECS caused a significant disruption of CTA only when the aversion was established with the weaker dose of LiCl. There was also no indication that interference with CTA was dependent upon close temporal contiguity between the ECS and LiCl. In a second experiment a CTA was established with LiCl (0.15 M) which was heated to 45 degrees C. Under these conditions ECS produced a similar disruption of learning to that when the UCS was the weaker dose of LiCl (0.075 M). The results suggest that an apparent differential loss of learning within the CS-UCS interval described in a previous report was accidentally created when some groups of animals were poisoned with warm and others with cold LiCl.     

Conditioned aversion to an intraperitoneally injected odor CS in rats]

Two experiments were conducted to specify an CS property of intraperitoneally injected odor substance in conditioned odor aversion formation. In experiment 1, three groups of water-restricted rats received one trial of conditioning with intraperitoneal injections of orange-extract (CS) and LiCl(UCS). The CS-UCS intervals of experimental groups were 30 minutes (Group IE-30) and 120 minutes (Group IE-120). In the test trial, Group IE-30 rats showed aversion to the test solution (0.5% orange extract), whereas no significant difference was observed between the control group and Group IE-120. In experiment 2, three groups of rats received one or two or three conditioning trials (Groups E1, E2 and E3, respectively). The CS-UCS interval of all groups was 30 minutes. In the test trial, rats in Groups E2 and E3 consumed less amount of test solution (0.5% orange extract) than the control rats, whereas no significant difference was observed between Group E1 and the control group. These results suggest that the intraperitoneally injected odor substance have CS property on conditioned odor aversion.     

Synergism of a compound unconditioned stimulus in taste aversion conditioning

Anti-lymphocyte serum (ALS) and lithium chloride have both previously been used successfully as unconditioned stimuli in taste aversion conditioning paradigms in rats. This report substantiates those findings but shows that when the two stimuli are given as a compound unconditioned stimulus in association with saccharin flavoured drinking solution, the conditioned taste aversion response following a second exposure to saccharin alone is more profound than that following conditioning with either ALS or LiCl alone. These results demonstrate synergism between the two stimuli when given together.     

Latent Inhibition and Conditioning in Rat Strains Which Show Differential Prepulse Inhibition

Latent inhibition (LI) is the retardation of associative conditioning resulting from preexposure of the conditioned stimulus (CS) alone prior to conditioning. Schizophrenic patients show deficient prepulse inhibition (PPI) and, at least acutely, deficient LI as well. We recently found that Brown Norway (BN) rats show a PPI deficit compared to Wistar-Kyoto (WKY) rats. If PPI and LI depend on neural processes with common genetic substrates, then LI should be deficient in BN rats as well. Here, LI of a conditioned taste aversion was examined in BN and WKY rats. One group from each strain was preexposed to a saccharin-flavored solution (CS) the day prior to conditioning. For taste aversion conditioning, these two groups again consumed saccharin and were injected with lithium chloride (unconditioned stimulus) 10 min later. A second group from each strain was not preexposed to the CS and was treated identically during conditioning, while a third group was not conditioned (injected with sodium chloride). To test for taste aversion conditioning, saccharin was offered for 20 min/day for 3 days. Nonconditioned BN and WKY rats consumed equal amounts of saccharin on test days. In both strains, conditioned rats showed a saccharin aversion. However, conditioning was less robust in BN than in WKY rats. WKY rats showed good LI of the conditioned taste aversion in that preexposed WKY rats consumed significantly more saccharin on test days than conditioned, nonpreexposed WKY rats. Preexposed BN rats did not consume significantly more saccharin on test days than conditioned, nonpreexposed BN rats. The previously reported deficiency in PPI in the BN rats was confirmed here 1 week after the taste aversion experiment. These results suggest that BN rats show deficient LI as well as PPI and display poor associative learning, a trait also reported in schizophrenia.     

Conditioned food aversion elicited by the temperature of drinking water as a conditioned stimulus in rats

It is known that taste can act as a conditioned stimulus (CS) for conditioned food aversion. In the present study, in order to examine whether or not the temperature of drinking water can be a CS, we conducted behavioral experiments in Wistar rats. The following results were obtained: (1) The rats subjected to aversive conditioning to 5 or 40 degrees C distilled water could learn to avoid these CSs, but they did not avoid any taste stimuli. (2) The rats subjected to aversive conditioning to 5 or 40 degrees C 0.1 M sucrose developed a generalized avoidance to sucrose at any temperature. (3) When rats familiarized to 25 degrees C 5 mM saccharin-Na (Sacc) were subjected to aversive conditioning to 5 or 40 degrees C Sacc, they avoided the respective CS, but they did not generalize it to any other stimuli even if having the same temperature as the CS. (4) The rats which had undergone transection of the taste nerves (chorda tympani and glossopharyngeal nerves) could acquire the conditioned response to the temperature of the CS. These results suggest that rats can be conditioned to temperature aversion and that the taste nerves are not needed in the formation of this conditioning.     

Involvement of the anterior insular gustatory neocortex in taste-potentiated odor aversion learning

When an odor conditioned stimulus (CS) precedes illness (unconditioned stimulus; UCS), rats acquire relatively weak odor aversions. Conversely, when a compound odor-taste (flavor) CS precedes illness, rats acquire robust aversions both to the odor and to the taste components of a compound flavor CS. Thus, tastes potentiate odor-illness aversions during toxiphobic conditioning. Such conditioning effects have been referred to as taste-potentiated odor aversion learning (POA). Previous neurobehavioral experiments have shown that the anterior insular gustatory neocortex contributes to conditioned taste aversion (CTA) learning. The present experiment examined the involvement of the anterior insular gustatory neocortex in CTA learning and POA learning. To that end, four distinct groups of rats received bilateral electrolytic lesion placements in the orbitofrontal neocortex, the "somatic" gustatory neocortex, the anterior insular gustatory neocortex or the posterior insular neocortex. Control animals received anesthesia only. Subgroups of animals thereafter received aversion conditioning using either an odor (almond) CS or a compound odor-taste (almond-saccharin) CS. Aversions to the almond odorant and/or saccharin tastant were evaluated during extinction. Results indicated that animals lacking orbitofrontal neocortex or posterior insular neocortex acquired normal CTAs and POAs. Animals lacking somatic gustatory neocortex exhibited impaired CTA learning, yet those animals showed normal POA learning. Lesions centered in the anterior insular neocortex impaired both CTA learning and POA learning. These results demonstrate that the insular gustatory neocortex is uniquely involved in the higher-order integration of odors, tastes and illness.     

Cycloheximide impairs reconsolidation of a contextually reactivated memory in a conditioned taste aversion paradigm

Rats were used to examine the impact of systemic protein synthesis inhibition (PSI) on the reconsolidation of a contextually reactivated memory of conditioned taste aversion (CTA). Rats were administered intraperitoneal injections of saline or lithium chloride (LiCl; .15 M) following exposure to a novel sucrose solution in a unique context. Seven days later, rats were injected subcutaneously with saline or cycloheximide (CXM; 1 mg/kg) and returned to their home cage or placed into the CTA training context in the absence of the target conditioned stimulus to reactivate the training memory. At testing, LiCl-trained rats that had been given CXM at reactivation had significantly greater difference scores (sucrose-water) in comparison with LiCl/CXM rats that had not been given a reactivation treatment and LiCl/saline memory-reactivated rats. These results suggest that context re-exposure effectively reactivates memory of CTA training that may be weakened through PSI. Extinction tests revealed rapid attenuation of taste aversions in all of the LiCl-injected groups. The involvement of taste-potentiated aversions and the role of the context in taste aversion conditioning are discussed.     

Conditioned diminution of the unconditioned skin conductance response

During Pavlovian conditioning the expression of a conditioned response typically serves as evidence that an association between a conditioned stimulus (CS) and an unconditioned stimulus (UCS) has been learned. However, learning-related changes in the unconditioned response (UCR) produced by a predictable UCS can also develop. In the present study, we investigated learning-related reductions in the magnitude of the unconditioned skin conductance response (SCR). Healthy volunteers participated in a differential conditioning study in which one tone (CS+) was paired with a loud white-noise UCS and a second tone (CS-) was presented alone. In addition, probe trials that consisted of UCS presentations paired with the CS+ (CS + UCS) and CS- (CS - UCS), as well as presentations of the UCS alone were included to assess UCR diminution. SCR and participants' expectations of UCS presentation were monitored during conditioning. Greater diminution of the UCR was observed to the UCS when it followed the CS+ compared to when it followed the CS- or was presented alone. Further, UCR amplitude showed an inverse relationship with the participants' ratings of UCS expectancy. However, conditioned UCR diminution was also observed independent of differential UCS expectancies. Our findings demonstrate conditioned diminution of the unconditioned SCR. Further, these findings suggest that although UCR amplitude is modified by conscious expectations of the UCS, conditioned diminution of the UCR can be expressed independent of learning-related changes in these expectations.     

Central gustatory lesions and learned taste aversions: unconditioned stimuli

The efficacy of two different unconditioned stimuli (US) in producing conditioned taste aversion (CTA) was tested in rats after bilateral ibotenic acid (IBO) lesions of the gustatory nucleus of thalamus (TTAx) and the medial and lateral parabrachial nuclei (mPBNx, lPBNx). An initial study determined an equivalent dose for the two USs, LiCl and cyclophosphamide (CY), using non-lesioned rats. Subsequently, using a separate set of lesioned animals and their sham controls (SHAM), injections of CY were paired 3 times with one of two taste stimuli (CSs), 0.1 M NaCl for half the rats in each group, 0.2 M sucrose for the other half. After these conditioning trials, the CS was presented twice more without the US, first in a 1-bottle test, then in a 2-bottle choice with water. The acquisition and test trials had 2 intervening water-only days to assure complete rehydration. Two weeks later, the same rats were tested again for acquisition of a CTA using LiCl as the US and the opposite CS as that used during the CY trials. The SHAM and TTAx groups learned to avoid consuming the taste associated with either CY or LiCl treatment. The two PBNx groups failed to learn an aversion regardless of the US.     

Effects of water deprivation and prior LiCl exposure in conditioning taste aversions

The phenomenon of attenuated taste aversion conditioning following preexposure to a lithium chloride UCS was demonstrated in Experiment 1. The amount of fluid consumed during the preconditioning phase was shown to be a factor in the degree of attenuated conditioning. The pharmacological properties of LiCl, and the effects of either ad lib or restricted fluid intake in conjunction with state dependent variables were proposed to account for the results. In Experiment 2 the water scheduling and LiCl habituation procedures of Experiment 1 were replicated, and serum lithium was assessed in the various groups on conditioning day. The groups did not differ, ruling out an incremental illness hypothesis of attenuated conditioning.     

Greater Resistance to Extinction of Electrodermal Responses Conditioned to Potentially Phobic CSs: A Noncognitive Process?

Previous results have suggested that electrodermal responses classically conditioned to potentially phobic CSs (e.g., pictures of snakes or spiders) are highly resistant to extinction and occur largely independently of cognitive expectancies. In order to test stringently for these possibilities, 144 college student subjects were administered differential classical conditioning acquisition and extinction paradigms while expectancies of the shock UCS were closely monitored. Half the subjects had potentially phobic CSs, whereas the other half had neutral CSs. Regardless of type of CS, during acquisition no evidence of electrodermal conditioning was found among subjects unaware of the CS?UCS contingency, nor was conditioning found on the pre-aware trials of subjects who became aware. During extinction, there was significantly greater resistance to extinction of electrodermal responses conditioned to potentially phobic CSs as well as a similar trend with expectancies of the UCS. However, when expectancies were equated, there was no greater resistance to extinction of electrodermal responses conditioned to potentially phobic CSs. Thus, while electrodermal responses conditioned to potentially phobic CSs did exhibit greater resistance to extinction, this conditioning was no more independent of expectancies than is conditioning with neutral CSs.     

Water-deprivation prevents morphine-, but not LiCl-induced, suppression of sucrose intake.

Intake of a saccharin-conditioned stimulus (CS) can be suppressed following pairing with an aversive agent such as lithium chloride (LiCl) or x-rays (referred to as a conditioned taste aversion or CTA), a highly rewarding sucrose solution (referred to as an anticipatory contrast effect), or a drug of abuse such as morphine or cocaine. Although the suppressive effects of LiCl and sucrose are clear examples of aversive and appetitive conditioning, respectively, it is not certain which properties (aversive or appetitive) mediate the suppressive effects of drugs of abuse. It is known, however, that the suppressive effects of a rewarding sucrose US are attenuated when using a caloric sucrose CS in food deprived rats, while LiCl induced CTAs are much less effected. Standard CTA testing typically is conducted in water-deprived rather than food-deprived rats and, although LiCl is known to suppress intake of a sucrose CS in water-deprived rats, the suppressive effects of drugs of abuse have not been evaluated under these conditions. The present experiment, then, compared the suppressive effects of a standard dose of morphine (15 mg/kg) and a matched dose of LiCl (0.009 M) on intake of a sucrose CS in water-deprived and free-feeding rats. The results showed that both drugs suppressed intake in free-feeding subjects, but only the aversive agent, LiCl, reduced CS intake in the water-deprived rats. This finding dissociates the suppressive effects of morphine and LiCl and, in so doing, aligns the suppressive effects of morphine with those of an appetitive sucrose US.     

Conditioned suppression of a thymus-independent antibody response

An illness-induced taste aversion was conditioned in mice by pairing cyclophosphamide, an immunosuppressive drug, with the consumption of saccharin, a novel drinking solution. Two weeks after conditioning, animals were injected with the hapten trinitrophenyl (TNP) coupled to the thymus-independent carrier, lipopolysaccharide. Serum antibodies to TNP were titered 6 days later by passive hemagglutination. Relative to control groups, conditioned animals provided with saccharin at the time of antigenic stimulation and, again, 3 days later showed a significant attenuation of their anti-TNP antibody response. In a second experiment, the conditioned stimulus (CS) consisted of the novel saccharin drinking solution plus the noxious internal effects of an injection of LiCl. Conditioned animals reexposed to the CS again showed the lowest antibody titers, but differed significantly from only one of the control groups. Taken together, the results of these experiments confirm previous reports of conditioned immunosuppression and suggest that the effects of conditioning on a primary humoral antibody response can be observed in response to a T-cell independent antigen in the mouse.